In health care, it is often desirable to analyze and monitor the gas composition of a patient's exhaled and/or inhaled breathing gases. For instance, measurement of respiratory CO2, O2, N2O, and anesthetic agents, such as halothane, isoflurane, enflurane, sevoflurane or desflurane, may be useful in the care of critically ill patients undergoing anesthesia. In some emergency care situations involving manual ventilation, it may typically be sufficient to monitor a patient's breathing with a simple CO2 analysis.
Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO2) in respiratory gases, and provides real-time information regarding CO2 exhalation and respiratory rates as well as a rapid and reliable assessment of a patient's ventilatory, circulatory and metabolic function. Although the terms capnography and capnometry are sometimes considered synonymous, capnometry suggests measurement without a continuous written record or waveform. Typically in capnography and capnometry, a gas analyzing device is placed in the respiratory circuit of a patient to sample exhaled and/or inhaled breathing gases and calculate gas concentrations directly in the respiratory circuit.
Measurement of end tidal CO2 can also provide useful information regarding CO2 production, pulmonary (lung) perfusion, alveolar ventilation, respiratory patterns, and elimination of CO2 from an anesthesia breathing circuit or ventilator. The gas sample measured at the end of a person's exhalation is called the “end-tidal” gas sample. The amount of CO2 in a person's end-tidal breath can indicate the overall efficiency of the cardio-pulmonary system and quality of breathing. For example, an overly high concentration of CO2 can indicate shallow breathing and poor oxygen intake. Thus, capnographs are used in hospitals and other medical institutions for monitoring the condition of a patient's respiratory system, pulmonary perfusion, and metabolism, and are often used for patients in intensive care or under anesthesia. Gas analyzers, including capnographs, can also be used in a wide range of other circumstances, for example ventilator management and weaning, metabolic measurements and nutritional assessment, and automated drug infusion safety.
The accuracy of the analysis of exhaled gases depends on the ability of a sampling system to move a gas sample from the patient to the gas analyzer while maintaining a smooth, laminar flow of gases, such that there are as few as possible alterations to the waveform representing the measured concentration of the gases. An accurate waveform depicting the concentration of the gas is critical for accurate patient monitoring and diagnosis.
Different types of oral/nasal cannulae are used to collect exhaled gas samples from patients in order to monitor respiration and other patient parameters. Some cannulae additionally deliver oxygen and/or other therapeutic gases, for example anesthetic gases, to the patient as needed.